Apheresis is a procedure in which individual blood components can be separated and collected from whole blood temporarily withdrawn from a subject. Typically, whole blood is withdrawn through a needle inserted into a vein of the subject's arm and into a cell separator, such as a centrifugal bowl. Once the whole blood is separated into its various components, one or more of the components can be removed from the centrifugal bowl. The remaining components can be returned to the subject along with optional compensation fluid to make up for the volume of the removed component. The process of drawing and returning continues until the quantity of the desired component has been collected, at which point the process is stopped. A central feature of apheresis systems is that the processed but unwanted components are returned to the donor. Blood components separated may include, for example, a high density component such as red blood cells, an intermediate density component such as platelets or white blood cells, and a lower density component such as plasma.
Among various blood component products obtainable through apheresis, the demand for plasma reduced platelet products is rapidly growing. This is particularly because, with the improvement in cancer therapy, there is a need to administer more and more platelets to patients with lowered hemopoietic function, but the same patients may not need to be transfused with the plasma used to suspend platelets. Platelets are fragments of a large cell located in the marrow called a megakaryocyte and primarily contribute to hemostasis by performing aggregation function. Platelets also have a role in tissue healing. Normal platelet counts in adults are 150,000-400,000/mm3. Platelet counts under 20,000/mm3 can cause various troubles such as spontaneous bleeding.
Platelets have a short half-life of 4-6 days and the number of donors is limited. Therefore, in producing plasma reduced platelet products, it is important to harvest platelets from the whole blood supplied by a donor at a maximum yield and in a required amount. Further, it is known that the contamination of plasma reduced platelet product by white blood cells can lead to serious medial complications, such as GVH reactions. Therefore, it is also very important to keep the level of contamination by white blood cells as low as possible, while efficiently collecting platelets. To this end, various techniques have been developed. For example, using “surge” technology, after whole blood is collected and concentrically separated within a centrifuge into higher density, intermediate density and lower density components and plasma is harvested (so-called “draw” step), plasma is supplied through the centrifuge at a surge flow rate (e.g., a flow rate that increases with time). By performing the surge, platelets can be preferentially displaced from the intermediate density components (which exist as a buffy coat mainly comprising a mixture of platelets and white blood cells), and plasma reduced platelet products can be produced at an increased yield.
Instead of using surge technology, the platelet layer can also be extracted from the centrifuge by means of a layer “push” in which anticoagulated whole blood is introduced into the bowl until the platelet layer is pushed out, or by using a combination of surge and push methodologies. After harvesting a desired component or components, the residual blood components mostly comprising red blood cells and citrated plasma are returned to the donor (so-called “return” step).
As mentioned above, in many blood apheresis procedures and applications, unwanted components (e.g., the components that are not collected) are returned to the donor. In addition to contamination concerns (e.g., contamination, particulates, etc. being returned to the donor), the comfort of the subject must be taken into consideration. For example, returning citrated plasma to the subject too quickly or returning too much citrated plasma to the subject at one time can cause significant discomfort to the patient. Additionally, care must be taken to limit the volume of fluid outside of the body (e.g., extra-corporeal volume) and/or the reduction of fluid in the body (e.g., intra-vascular deficit).